A process of performing a processing of a substrate such as, for example, a semiconductor wafer (hereinafter, referred to as a “wafer”), by supplying a processing liquid to the substrate, includes sequentially switching and supplying different kinds of processing liquids. For example, when a rinse liquid such as, for example, deionized water (DIW), is supplied to a wafer to perform a rinse processing after the wafer is processed with a chemical liquid, or when highly volatile isopropyl alcohol (IPA) is supplied to dry the wafer after the rinse processing, it is required to sufficiently replace the processing liquid (first processing liquid) existing on the wafer surface with a processing liquid (second processing liquid) to be supplied later.
With respect to the replacement operation of the processing liquids, it is required to determine whether the processing liquid (first processing liquid) existing on the wafer surface is replaced with the processing liquid (second processing liquid) to be supplied later, in view of consumption amounts of the processing liquids or processing time. For example, Japanese Patent Laid-Open Publication No. 2009-218402 (paragraphs [0028] to [0031] and FIG. 1) discloses a technology in which when DIW (rinse liquid) supplied to a rotating wafer is replaced with IPA, it is detected whether the DIW is replaced with the IPA, by making a prism in contact with a liquid surface of the IPA formed on the surface of the wafer to cause light emitted from the prism to pass through the IPA, be reflected, and be received, and calculating a water concentration based on attenuation of a predetermined wavelength component.